Expression of the first 811 nucleotides of the herpes simplex virus type 1 latency-associated transcript (LAT) partially restores wild-type spontaneous reactivation to a LAT-null mutant

The herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) gene is required for efficient spontaneous reactivation in the rabbit ocular model. We recently showed that insertion of 1.8 kb of the LAT promoter and the first 1.5 kb of the 8.3-kb primary LAT transcript into a novel, ectopic location in the virus unique long (UL) region restored wild-type spontaneous reactivation to a LAT-null mutant. To further map the LAT spontaneous reactivation function within the first 1.5 kb of LAT, we rescued the same LAT-null mutant by inserting 1.8 kb of the LAT promoter and just the first 811 nucleotides of LAT into the same location in the UL. In a series of three experiments, the resulting virus, designated LAT2.6A, had a spontaneous reactivation rate that was midway between the original LAT-null mutant and wild-type virus. Thus expression of the first 811 LAT nucleotides produced a spontaneous reactivation rate that was significantly higher than that of the LAT-null mutant but significantly less than that of wild type. This suggests that part, but not all, of the LAT function involved in efficient spontaneous reactivation is located within the first 811 nucleotides of the primary 8.3- kb LAT.